Johnson made his debut as a professional boxer in 1989 following a successful amateur career, during which he fought in over 130 contests. As a professional he quickly built on his reputation as a top prospect by remaining undefeated after his first 22 contests (one of which was drawn). His next fight was against another highly rated youngster, Sharmba Mitchell (who was also undefeated as a professional). Johnson won the fight, earning himself his first world title shot, by knocking his opponent out in the eighth round. The world title fight came on August 6, 1994 against Miguel Ángel González for the WBC Lightweight crown. The fight, which was fought in Ciudad Juárez, Mexico, ended with Johnson suffering his first loss as a professional, the fight being stopped in the eighth round

Johnson got his second world title shot in May 1997, this time challenging Orzubek Nazarov for the WBA version of the lightweight world title. Johnson's challenge once again proved unsuccessful as the fight was stopped in the seventh round for his second loss. He did not fight in another world championship fight for six and a half years after this, during which time he lost once more after he was disqualified for headbutting Michael Clark during a contest in Atlantic City. After losing to Javier Jauregui in a fight for the vacant IBF world title in November 2003, he only fought once in the eighteen months after before getting another shot at the IBF crown, against Zoff. Johnson won the fight, in what proved to be the highlight of his career.

Johnson defended his title for the first time on September 17, 2005, against Mexican fighter Jesús Chávez. The fight was stopped early in the 11th round after Johnson received a barrage of punches from his opponent. Despite being able to leave the ring under his own momentum, he collapsed in his dressing room shortly afterwards. He was rushed to the hospital and had emergency surgery to correct a subdural hematoma (swelling and bleeding on the brain). Following the surgery, Johnson was placed in a drug-induced coma. Despite initial fears that he would not survive the night even after the surgery, he survived into the next week and was showing some early signs of improvement, but still remained in critical condition. However, his condition stopped improving and on September 22, 2005, it was decided to discontinue efforts to artificially prolong his life. He was pronounced dead at 4:23 p.m. PDT surrounded by his family.^[2]

Johnson's career record was 34 wins (26 by KO), 5 losses, 2 draws, and 1 no contest.

Johnson's death, along with a contest earlier in the year, led to the Nevada Athletic Commission imposing new rules in 2006.

Rule changes were made regarding hydration of boxers, which includes the legalisation of sealed sports beverages with electrolytes in a venue, mandatory tests on nutrition, concussions, weight loss, and hydration for all trainers and seconds, heavier glove requirements (ten-ounce gloves required for 135 pounds and heavier boxers), the use of three ringside physicians (one for each boxer and a third) during each match, and new mandatory inspections of competitors during the weigh-in, before the contest, and immediately after the contest. As of 2024, no boxer has been killed in a professional boxing contest in Nevada since Johnson's death^[3].

• Braydon Smith 23-year-old featherweight collapsed 90 minutes after match and died days later.

• Boxing record for Leavander Johnson from BoxRec (registration required)
• Leavander Johnson Foundation official site